1. Field of the Invention
This invention relates to a resin mirror-drum for use in cameras such as photographic cameras, VTR cameras, monitor cameras, and the like, and to a method for the manufacture thereof.
2. Description of the Prior Art
In the recent years, camera models have been improved by the addition of autofocussing function and a zoom function; in particular, the resolution of VTR cameras is being increased. On the other hand, cameras are needed to be made at low cost, and the mirror-drums that are essential to attain these functions are more frequently being made of resin.
Polycarbonates are the main kind of resin used for the mirror-drum; at times, polyphenylene oxide (NORYL; a trademark of EPL Co.), or where particular precision is not needed, polybutylene terephthalate or the like, are used. With these resins, glass fibers with the diameter of 10-20 .mu.m, or at times, carbon fibers with the diameter of 2-10 .mu.m, are admixed as fillers to increase the strength of the resins and also to maintain fixed measurements over a long period of use.
However, resin mirror-drums are not used in high-quality cameras. The mirror-drum of high-quality cameras is made of aluminum. The reason is described below with a zoom lens for typical VTR cameras:
FIG. 7 shows a zoom lens for a conventional VTR camera. Lenses 15, 16, and 17 that form a group of lenses are fixed onto a focussing-lens frame 18. An external helicoid thread 19 that is formed in the focussing-lens frame 18 fits into an internal helicoid thread 21 that is formed on the camera body 20. A cam frame 22 is rotatably disposed inside of the body 20. A cam follower 23 fits into cam grooves of the cam frame 22. The cam follower 23 is fastened to movable frames 24 and 25 so as to be one piece with them. These movable frames 24 and 25 are guided by a guide pole 30 in the direction of the optical axis. To the movable frame 24 there are fixed lenses 26, 27, and 28, which form a group of lenses for changes in magnification, to the movable frame 25 there is fixed a compensating lens 29. To the body 20, a lens 31 and a master lens-frame 32 are fixed. Reference numerals 33, 34, 35, and 36 are lenses that form the master lens group; and reference numeral 37 is the CCD image sensor that changes images into electrical signals.
The above-mentioned construction must be designed so that the focussing-lens frame 18 and the cam frame 22 can smoothly rotate inside of the body 20, and moreover, the lenses must be supported so as not to become inclined. For that reason, the following conditions are essential: (1) that the roundness of each part and the precision of the shapes of the screws be excellent; (2) that the hardness of the body 20 be sufficient that a slight external force should not cause deformation; and (3) that the thermal expansion coefficient of the body 20 and of each of the frames 18 and 22 be close to that of the lenses, so that even at high or low temperatures, the slight clearance between the inside surface of the body and each of the frames be maintained so that each of the frames can rotate smoothly. However, the above-mentioned resins that contain fibers as a filler (i.e., fiber-reinforced plastics) are inferior to aluminum in all three of these points, so they cannot be used in high-quality cameras.
Therefore, so that the functions and the qualities of cameras can be improved and so that the cameras can be manufactured at low cost, a plastic that is satisfactory in these three points is needed.
The following is already known about fiber-reinforced plastics: (1) the hardness of plastics increases as the amount of fibers used for filling increases; (2) the phenomenon described in the preceding item 1 is more marked as the fiber length increases, but there is not further increase in the phenomenon when the fibers have above a certain aspect ratio (length of fiber/diameter of fiber); (3) the thermal expansion coefficient of plastics increases as the amount of fibers used for filling increases; (4) the phenomenon described in the preceding item 3 is greater in the direction of the orientation of the fibers as the fiber length increases, but in the direction at right angles to the orientation of the fibers, there is almost no such effect; and (5) the effect of the orientation of the fibers increases as the fiber length increases, and accurate molding becomes difficult.
It is generally decided from the above, that the amount of glass fibers for fiber-reinforced plastics for use in camera mirror-drums should be 20 to 30% by weight (11-17% by volume), and that the aspect ratio should be 10-30.
These fiber-reinforced plastics can be made by the following process. Extremely long glass-fiber roping is chopped first to lengths of about 6 mm, and the chopped glass obtained and a matrix polymer (a mixture of polycarbonate, polyphenylene oxide, or the like, and small amounts of a stabilizer, coloring agent, lubricant, and other ingredients) are weighed and blended at specified proportions; then the mixture is kneaded by a screw-type extruder as shown in FIG. 8. The blended material supplied to a hopper 38 is conveyed onward into a cylinder 41 by a screw 40 driven by the rotation of a motor 39. The matrix polymer is fused by the heat of cylinder heaters 42 and the that given rise to by friction and shearing during the rotation of the screw 40. The chopped glass inside the screw channel is put under considerable pressure, and is broken up and dispersed into the molten matrix. Because of the coupling of the glass fibers beforehand, this dispersion takes place smoothly, and the strength of the composition obtained is satisfactory.
The depth of the screw channel of the screw 40 gradually decreases toward the tip of the screw 40, and the shape of the screw is designed so as not merely to convey the blended material forward but so as to help prevent flowing. Accordingly, the glass fibers that are dispersed in the polymer are chopped efficiently.
There is a strand die 43 at the top of the cylinder 41 from which is extruded the molten plastic composition in the shape of a rope. The extruded plastic is cooled in a water-bath 44, and the cooled and hardened plastic is cut fine by cutter 45 into pellets with length of 2-5 mm. This is used as a material for molding.
At this stage, the aspect ratio of the fibers in the material is kept as a value close to that of the final product. However, by the above method, the length to which the glass fibers are cut is not directly decided; cutting depends on the shearing stress at the time of movement forward and kneading depends on the shape of the screw, and there is a range of lengths of the fibers from short to long; manufacturing conditions are decided so that the mean length will generally be within the desired range.
If the glass fibers are chopped so as to be close to the length of the final aspect ratio before the introduction of the material into the hopper 38, the aspect ratio will be very uniform, so it should be possible to obtain glass fibers with a stable mean aspect ratio. However, when the aspect ratio is to be 10-30, it is necessary to use a cutter with a knife set at a pitch of about 0.1-0.6 mm. Cutters that cut glass are readily damaged by abrasion, so they are designed so as to be replaceable. For this reason, it is difficult to achieve this kind of construction to cutters with the above-mentioned pitch.
The material for molding formed in this way is dried sufficiently and supplied to an injection molding apparatus, and a camera mirror-drum is molded.
FIG. 9 is a part of an injection molding apparatus in which the material for molding is supplied to a hopper 46, and by the rotation of a screw 47, the material is supplied toward the front of a cylinder 48 while being heated. While the molten material is being moved forward by the screw 47 that is driven by an injection cylinder 50, it is injected into a mold 49, where the resulting molded product is cooled. Then it is removed from the mold 49.
FIG. 10 shows the screw 47 of the injection molding apparatus of FIG. 9, which generally is a full-flight screw, in which the screw channel is uniformly arranged, unlike the screws of the extruders for kneading. The depth of the screw groove gradually decreases from the hopper side thereof, with the shallowest part close to the tip thereof; the ratio d.sub.1 /d.sub.2 and d.sub.1 is the depth of the groove near the hopper and d.sub.2 is the depth of the groove near the tip is called the constriction ratio. The value of this ratio is generally about 2-3; the larger this value, the better is the kneading that is achieved, and the debubbling effect is also good. However, some glass fibers with a large aspect ratio are cut when passing through this screw groove, and some are also cut when being injected into the mold. Accordingly, the aspect ratio of the glass fibers in the final product varies depending on the conditions of the rotation of the screw and on the injection conditions. Therefore, the conventional method and apparatus described above raise the following problems: (1) because the lengths of the fibers differ, the material for molding has different contraction coefficients, and the measurements of products obtained by the use of the same mold differ, causing difficulty in fitting, for example, one screw product into the other; and (2) products with different accuracy of molding are obtained, and it is not possible to ensure the stable positioning of the lenses.